Which part of the phospholipid molecule is hydrophilic?

In a phospholipid molecule, the part that is hydrophilic is the polar head. This portion is composed of a phosphate group and is charged, which allows it to interact favorably with water molecules. The polar head's affinity for water (hydrophilic nature) is critical for the formation of biological membranes, where phospholipids arrange themselves into a bilayer with the hydrophilic heads facing the aqueous environment on both the outside and inside of the cell, while the hydrophobic fatty acid tails face inward, away from water.

Understanding this arrangement is essential for grasping how cell membranes function in terms of permeability and the selective transport of substances. The ability of the polar head to form hydrogen bonds with water molecules is a key characteristic that distinguishes it from the hydrophobic fatty acid chains, which do not interact positively with water. Thus, the polar head is crucial for the structural integrity and functionality of cells in an aqueous environment.